Quantification of total carbon dioxide species, i.e., carbon dioxide (CO2), bicarbonate (HCO3), and carbonate (CO3), can be applied to various fields including those related to process analysis, environmental analysis, and in clinical analysis. For example, it can be applied for analyzing clinical samples in the medical diagnostic fields, for regulating combustion processes in chemical analysis fields, for diagnosing the severity of the greenhouse effect, and for measuring the indexes related to aquatic ecosystem in an environmental field. The exact measurement of dissolved carbon dioxide becomes increasingly significant.
In a human body, determination of the total carbon dioxide species is related to the acid-base and electrolyte balance in the human body which is regulated by pulmonary and renal mechanisms. The main ion connecting these two regulative pathways is the bicarbonate ion (HCO3−). The malfunction in either of these two mechanisms is manifested by changes in the bicarbonate concentration accompanied by changes in pH and dissolved partial pressure of carbon dioxide, or pCO2. The free carbon dioxide is dissolved in blood plasma and hydrated to form carbonic acid (H2CO3), which is in turn dissociated into hydrogen ion (H+) and bicarbonate ion (HCO3+). Since the total content of carbon dioxide (CO2 gas, H2CO3, HCO3−, CO32−) in blood plasma affects the acid-base balance and pH of blood, as well as being an index for pulmonary ventilation, alveolar gas exchange capacity, and the quantity of the gas transferred to somatic tissues from blood, it is very important to accurately measure the levels of carbon dioxide dissolved in blood. The value of total CO2 in biological fluids such as human blood or serum or plasma can be calculated if pH and the concentration of one form of CO2 (pCO2, HCO3 or CO3) are measured as all are linked by Henderson Hasselbalch equation.
One way to measure concentrations of carbon dioxide is to use a Severinghaus-type carbon dioxide gas sensor, wherein an external reference electrode, a pH-sensitive working electrode and a gas-permeable membrane are simultaneously housed in one sensor body. The Severinghaus-type carbon dioxide gas sensor is immersed in internal filling solution (IFS), which is unbuffered. CO2 permeates through the gas permeable membrane and dissolves in the internal filling solution, thereby changing the pH of the IFS. This change in pH is directly proportional to the pCO2. The Severinghaus-type carbon dioxide gas sensor has a mechanically complicated structure and suffers interference from volatile organic acids. Another disadvantage of the Severinghaus-type carbon dioxide gas sensor is difficulty in miniaturization of the sensor, because the reference electrode is incorporated inside the sensor body. Further, the Severinghaus-type carbon dioxide gas sensor suffers from the disadvantage of being poor in detection limit.
Another way to measure concentrations of carbon dioxide is by use of a differential-type carbon dioxide gas sensor, wherein a working electrode and a reference electrode are separated in different sensor bodies. The differential-type carbon dioxide gas sensor comprises a working electrode composed of an unbuffered inner reference solution and a pH-sensitive gas-permeable membrane; and a reference electrode composed of a buffered inner reference solution and the same pH-sensitive gas-permeable membrane as that in a working electrode.
In the differential-type carbon dioxide gas sensor, charge separation and the accompanying potential difference occur at 4 different interfaces: E (ext1) between the pH-sensitive gas-permeable membrane of the working electrode and the sample solution; E (ext2) between the pH-sensitive gas-permeable membrane of the reference electrode and the sample solution; E (int1) between the pH-sensitive gas-permeable membrane of the working electrode and the unbuffered inner reference solution; and E (int2) between the pH-sensitive gas-permeable membrane of the reference electrode and the buffered inner reference solution.
When such charge separations occur, E (ext1) and E (ext2) have the same value and thus can be counterbalanced, as the same pH-sensitive gas-permeable membranes are used. On the other hand, the charge separation E (int2) between the pH-sensitive gas-permeable membrane of the reference electrode and the buffered inner reference solution is maintained at a constant value as the reference solution is buffered. Therefore, a change in carbon dioxide levels in a sample solution causes only the charge separation E (int1) between the pH-sensitive gas-permeable membrane and the unbuffered inner reference solution of the working electrode, so that the resulting potential change enables the carbon dioxide levels of the sample solution to be quantitatively detected.
A promising approach is to use a carbonate-selective electrode, which usually employ trifluoroacetophenone derivative as neutral carriers for carbonate. However so far the applications of carbonate selective electrodes have been limited by insufficient sensitivity and selectivity of the electrodes. The sensitivity problem is usually dealt with by increasing the pH of the sample, or pretreatment of sample, which is not possible on an analyzer, such as a blood gas analyzer. Traditionally, multiple formulations have been proposed to use the carbonate-selective electrode in series with other ion-selective electrodes commonly employed in automated clinical chemistry/electrolyte analyzers by reduced interference from anions such as salicylate in the measurement range of 5-50 mm total CO2 at physiological pH without pretreatment of the sample. Normal salicylate level in blood are typically less than 0.1 mmol/L, but therapeutic levels for aspirin users are approximately 1.5 mmol/L. Formulations with higher selectivity towards Salicylate could cause substantial positive errors in total CO2 measurements. One of the existing approach describes a buffer overcoat layer which is supposed to eliminate organic anion interferences.
Therefore, there exists a need for a robust arrangement of differential carbon dioxide gas sensors and related systems and methods that prevent elevated CO2 signal within therapeutic levels of salicylate in biological fluids.